The invention relates generally to a method of manufacturing boat parts submerged when in use (i.e. so called "wet" or underwater parts) such as rudder plates, propellers, lower units of outboard motors and of inboard-outboard motors etc., and to parts manufactured by the method.
Conventionally, such parts are produced of metallic materials such as iron, zinc or aluminum by molding or diecasting and they are, depending on the material used, provided with several surface conditioning layers of varying character, such as zinc coatings, chromating layers, primer layers, paint coatings etc. To apply a plurality of such layers is expensive, among other things also because relatively long waiting times may be necessary between two subsequent applications, to allow the underlying layer first to dry properly.
However, in spite of the presence of a plurality of surface conditioning layers, it cannot be excluded that after a certain time of service, or when hitting an underwater obstacle, a defect or fault arises exposing a bare metallic surface to water. Thereby arises the risk of corrosion by an electroerosive process taking place between the exposed spot and some other submerged bare metallic part, even if the involved metals may not be susceptible to corrosion by oxidation. Not even sailing boats with hulls of wood or plastics are exempt, because they mostly have keels comprising ballast metal (iron) and electroerosion may occur between this metal and e.g. an auxiliary outboard motor made mainly of aluminum.
It will be readily understood that submerged parts of a boat must be as much streamlined as possible. Said conventional surface conditioning layers, irrespective of their character and number, can always create only a uniformly thick coating on the underlying metallic surface, so that this surface in general must have the final shape of the respective part called for by hydrodynamic reasons, i.e. a streamlined shape. Moulding and diecasting processes do not allow greater differences in wall thickness (and such differences in a metallic object would also cause a considerable increase in weight) so that the inner walls of all underwater parts having a cavity, such as the lower units of outboard or inboard-outboard drive units, must generally follow the streamlined shape of the outer walls. Because of this, relatively complex mandrels or mold cores, often more than one, must be used in order to obtain the necessary streamlined outside shape of a casting which then may be covered by uniformly thick surface conditioning coatings.
Attention is directed to German patent specification No. 360952, issued Oct. 9, 1922, to Zeppelin-Werke G.m.b.H. and Albert Lehrle, and which discloses a settable ship's propeller of aluminum, and to Swiss patent specification No. 527680, issued Sept. 15, 1972, to Societe Nationale Industrielle Aerospatiale, which discloses a method of covering a structure having a cavity with a layer of a liquid substance capable of being hardened.